The formation of fibers and nonwoven webs by meltblowing is well known in the art. See, by way of example, U.S. Pat. No. 3,016,599 to R. W. Perry, Jr.; U.S. Pat. No. 3,704,198 to J. S. Prentice; U.S. Pat. No. 3,755,527 to J. P. Keller et al.; U.S. Pat. No. 3,849,241 to R. R. Butin et al.; U.S. Pat. No. 3,978,185 to R. R. Butin et al.; U.S. Pat. No. 4,100,324 to R. A. Anderson et al.; U.S. Pat. No. 4,118,531 to E. R. Hauser; and U.S. Pat. No. 4,663,220 to T. J. Wisneski et al.
Briefly, meltblowing is a process type developed for the formation of fibers and nonwoven webs; the fibers are formed by extruding a molten thermoplastic polymeric material, or polymer, through a plurality of small holes. The resulting molten threads or filaments pass into converging high velocity gas streams that attenuate or draw the filaments of molten polymer to reduce their diameters. Thereafter, the meltblown fibers are carried by the high velocity gas stream and deposited on a collecting surface, or forming wire, to form a nonwoven web of randomly dispersed meltblown fibers.
Generally, meltblowing utilizes a specialized apparatus to form the meltblown webs from a polymer. Often, the polymer flows from a die through narrow cylindrical outlets and forms meltblown fibers. The narrow cylindrical outlets may be arrayed in a substantially straight line and lie in a plane which is the bisector of a V-shaped die tip. Typically the included angle formed by the exterior walls or faces of the V-shaped die tip is 60 degrees and is positioned proximate to a pair of air plates, thereby forming two slotted channels therebetween along each face of the die tip. Thus, air may flow through these channels to impinge on the fibers exiting from the die tip, thereby attenuating them. As a result of various fluid dynamic actions, the air flow is capable of attenuating the fibers to diameters of from about 0.1 to 10 micrometers; such fibers generally are referred to as microfibers. Larger diameter fibers, of course, also are possible depending on polymer viscosity and processing conditions, with the diameters ranging from around 10 micrometers to about 100 micrometers.
Investigation has been done in the art with respect to the effect of varying certain parameters of the attenuating air flows. For example, U.S. Pat. Nos. 6,074,597 and 5,902,540 disclose a meltblowing method and apparatus utilizing a die assembly formed from a stack of laminated plates having aligned orifices that define an adhesive flow path flanked on each side by air flows. The adhesive flow is drawn and attenuated by the air flows. These patents allege that convergent air flows in the conventional V-shaped die assemblies are inefficient, and that the air flows should be non-convergent with respect to the adhesive flow to maximize the shear component of the compressed air flows.
U.S. Pat. No. 6,336,801 discusses the advantages of using as a primary drawing medium attenuating air that is cooler than the temperature of the polymer within the die tip and exiting from the nozzle outlets. One advantage is that the fibers quench more rapidly and efficiently, resulting in a softer web and less likelihood of formation of undesirable shot. (“Shot” is the accumulation of molten polymer at the die tip apex that eventually reaches a relatively large size and is blown from the die nose, not as a fiber, but as a blob or “shot.”) Another advantage is that faster quenching may reduce the required forming distance between the die tip and the forming wire, thereby permitting the formation of webs with better properties, such as appearance, coverage, opacity, and strength. The '801 patent describes a novel die assembly that focuses heat at the die tip to maintain a desired polymer viscosity and thereby permitting use of significantly cooler attenuating air.
The art is continuously seeking ways to improve the meltblowing process to maximize efficiency and provide an improved meltblown web. The present invention relates to an improved die tip assembly for this purpose.